Interpretive Summary: Water and nitrogen are essential for productive broccoli growth and yield. Timely applications of water and fertilizer to the crop avoid plant stress, which can reduce crop yield. They also help avoid over-application of water and nitrogen, which can potentially harm the environment. However, to determine the best time to make these applications during the season, the grower must know the broccoli’s nitrogen and water status. This study compared various crop indices that are based on remote sensing measurements to determine whether they can give a reliable indication of nitrogen and water status of broccoli. Several common vegetation indices that were studied could successfully "see" nitrogen and water treatment differences in broccoli growth, but these could not differentiate between water and nitrogen stress. However, an index called the Canopy Chlorophyll Content Index (CCCI) was able to detect nitrogen stress, while an index called the crop water stress index (CWSI) could detect water stress. This study indicates that combining information from remotely-sensed vegetation, nitrogen, and water stress indices may provide a dependable way to indicate both the nitrogen and water status of broccoli. This could help growers make better-informed management decisions about nitrogen and fertilizer applications during the season. This research will be of interest to farmers, farm advisors, and government agencies.

Technical Abstract:
Remote sensing (RS) can facilitate the management of water and nutrients in irrigated cropping systems. Our objective for this study was to evaluate the ability of several RS indices to discriminate between limited water and limited nitrogen induced stress for broccoli. The Agricultural Irrigation Imaging System (AgIIs) was used over a one-hectare broccoli field in central Arizona to measure green, red, red edge, and near infrared (NIR) reflectances, and thermal infrared radiation. Measurements were taken at a 1x1 resolution, every several days during the season. From, the data, the following indices were calculated: ratio vegetation index (RVI), normalized difference vegetation index (NDVI), normalized difference based on NIR and green reflectance (NDNG), canopy chlorophyll concentration index (CCCI), and the crop water stress index (CWSI). The experimental design was a two-factor, nitrogen x water, Latin square with four treatments (optimal and low water and optimal and low nitrogen) and four replicates. In addition to RS measurements, the following in-situ measurements were taken: SPAD chlorophyll (closely related to nitrogen status), plant petiole nitrate-nitrogen concentrations, soil water content, and plant height, width, and leaf area index (LAI). Fresh marketable broccoli yield was harvested from plots 131 days after planting.
Seasonal water application (irrigation plus rainfall) was 14% greater for optimal than low water treatments, whereas total nitrogen application was 35% greater for optimal than low N treatments. Although both nitrogen and water treatments affected broccoli growth and yield, nitrogen effects were much more pronounced. Compared to the optimal water and nitrogen treatment, broccoli yield was 20% lower for low water but optimal nitrogen, whereas yield was 42% lower for optimal water but low nitrogen. The RVI, NDVI, and NDNG indices detected treatment induced growth retardation but were unable to distinguish between the water and nitrogen effects. The CCCI, which was developed as an index to infer differences in nitrogen status, was found to be highly sensitive to nitrogen, but insensitive to water stress. The CWSI effectively detected differences in water status following several irrigation events when water was withheld from low water treatments but not optimal ones. However, the CWSI incorrectly signaled high water stress for low nitrogen treatments, which occurred before the beginning of irrigation treatments, and water stress for optimal irrigation treatments prior to complete cover. Using a RS ground-based monitoring system to simultaneously measure vegetation, nitrogen, and water stress indices at high spatial and temporal resolution could provide a successful management tool for differentiating between the effects of nitrogen and water stress in broccoli.